1. Field of the Invention
The present invention relates to a mobile communication method and a mobile communication system wherein each of a plurality of large service area base stations covers one of a plurality of large zones into which the service area is divided, each of a plurality of small service area base stations covers one of a plurality of small zones into which each large zone is divided, and the radio channel switching for mobile terminals used in the service areas of these large area or small area base stations is controlled by a mobile switching center.
2. Description of the Related Art
In a conventional mobile communication system typically of the cellular type, the radio zone to be covered by each base station is made a small size for increasing the capacity of subscribers through improvement of the efficiency of frequency utilization, i.e. the same frequency can be used in some other zones in which the same frequency does not interfere each other. However, this involves the disadvantage that the smaller the radio zones, and the faster, the mobile terminal moves, the more frequent the shift of the mobile terminal from one radio zone to another, resulting in such deterioration of service quality based on increased momentary interruption or more lasting interruption of communication due to channel switching.
To overcome this disadvantage, there has been invented a method by which each service area is divided into a plurality of large zones, each of which is further divided into a plurality of small zones, and the use of the large and small zones is differentiated according to the moving speed of the mobile terminal (see the Japanese Patent Laid-Open Gazette No. 1990-244917). FIG. 6 is a flow chart showing how this type of radio zone selecting system operates. Thus, the radio zone to be used by the mobile terminal, when the quality of communication deteriorates at the time of originating a call, terminating a call or during communication, is determined according to a table provided at the mobile switching center for determination of the radio zone to be used correspondingly to the moving speed and according to moving speed information collected from the mobile terminal.
According to the conventional radio zone selection method described above, because the mobile switching center selects the radio zone, the expansion of the subscriber capacity by decreasing the size of each radio zone may be restricted by the processing capacity of the mobile switching center. Moreover, since the radio channel is not switched until the quality of communication deteriorates even if the moving speed varies during communication, if the moving speed varies from low to high during communication, the subsequent channel switching may prove faulty.
An object of the present invention, with an eye to solving these problems, is to alleviate the drop in the processing capacity of the mobile switching center and the deterioration of service quality due to decreasing the size of radio zones by having the mobile terminal take the initiative in selecting the radio zone and switching the channel during communication by selecting the radio zone according to its moving speed.